docs: def end_container(self, header_buf):
          """Add a node containing the container's header to the current subtree.
  
          This node will be added as the leftmost leaf of the subtree that was
          started by the matching call to start_container.
  
          Args:
              header_buf (bytearray): bytearray containing the container header.
          """
          if not self.__container_nodes:
              raise ValueError("Attempted to end container with none active.")
          # Header needs to be the first node visited on this subtree.
          self.__container_node.add_leaf(_Node(header_buf))
          self.__container_node = self.__container_nodes.pop()
          parent_container_length = self.__container_lengths.pop()
          self.current_container_length = \
              parent_container_length + self.current_container_length + len(header_buf)